It is proposed to continue investigations of the biosynthesis of three novel, naturally occurring cyclopentanoid compounds. These are the alkaloid sesbanine, which occurs in the leguminous plant Sesbania drummondii, the nucleoside antibiotic aristeromycin, which is produced by Streptomyces citricolor, and the phytotoxin coronatine, which is isolated from Pseudomonas syringae. Previous studies have revealed that sesbanine is derived from nicotinic acid and shikimic acid with the possible intermediacy of p-hydroxybenzoic acid. The goal of future studies will be to verify that p-hydroxybenzoic acid is a specific precursor of sesbanine and to examine the mechanism of the conversion of the aromatic ring of this compound into the cyclopentane ring of sesbanine. Prior investigations of aristeromycin biosynthesis have shown that the adenine ring of the antibiotic is biosynthesized along familiar lines while the cyclopentane ring of the antibiotic is derived from D-glucose. Some clues to the mechanism of the ring closure were obtained by means of incorporation experiments with specifically tritiated forms of glucose. In the future, the studies with specifically tritiated forms of glucose will be completed and efforts will then be focused on elucidating the nature of some of the intermediates involved in the formation of the cyclopentane ring. These latter investigations will utilize precursor incorporation experiments, cell-free extracts, and mutants. Initial investigations of coronatine have demonstrated that the hydrindane portion of the molecule, coronofacic acid, is a novel polyketide derived from five acetate units and one pyruvate unit, with the latter serving as the starter unit. The cyclopropyl amino acid moiety of the toxin, coronamic acid, has been found to be biosynthesized from L-alloisoleucine. Future investigations of coronatine bio- synthesis will employ precursor incorporation experiments to probe both the mechanism of assembly of the polyketide and the mechanism of cyclopropane ring formation.